1. Field of the Invention
This invention concerns a postal card and, more specifically, it relates to a postal card having a cover sheet releasably attached on a printed card substrate.
2. Description of the Prior Art
Postal cards are used most popularly among various types of mail matters for delivering simple messages or communications since they are inexpensive and easy to handle with.
However, since the messages written on a postal card can be easily read by stealth by other persons than the addressee, it is not so favored in the case of private letter. Also in the case of business communication in which the privacy of individual clients has to be protected, such as a notification of a bank balance, tax paper, etc., papers containing relevent information are usually mailed being sealed in envelopes.
Sealed letters, however, need higher cost and more complicate labors, for example, packing of papers into envelopes, sealing of envelopes, etc. as compared with postal cards, which are inegligible in government and public offices, as well as private companies where a great amount of papers are dispatched daily on mail. Particularly, as computer-controlled printers have gradually been introduced in recent years for printing such mail matters, conveniency of simple postal cards suitable to such line-printing work has now been estimated highly and the advent of those cards capable of keeping the secret of correspondence has been desired.
In view of the above, a postal card of a laminated structure in which a cover sheet is releasably appended to a card substrate on which messages are printed or written has been proposed (Japanese Utility Model Laying Open No. Sho 62-9571, Composite Attachment to Postal Card). In such a postal card, a transparent protection film is bonded to the printed surface of a card substrate and a cover sheet having a hiding power is releasably attached to the surface of the transparent film.
Thus, the printed area on the surface of the card substrate such as for the sum of bank balance is concealed by the cover sheet. Then, only when the addressee receiving the postal card releases the cover sheet, the printed content can be read through the transparent protection film.
However, this kind of releasable postal card still involves several problems in actual use.
At first, the cover sheet has to be bonded firmly so that it is never turned up from the card substrate till the postal card is finally received by the addressee. If the adhesion of the cover sheet to the card substrate is weak, it easily peels off, failing to attain the purpose of keeping written message secret, as well as post office may reject the acceptance of such troublesome mail matters.
While on the other hand, the cover sheet has to be released easily and smoothly from the card substrate by the addressee receiving the card. If the adhesion of the cover sheet is too strong, the cover sheet itself may be partially torn or the printed surface of the card substrate is plucked off upon releasing, to hinder easy and clear reading of the printed content.
These conflicting requirements may be solved, for example, by using a strong adhesive between the protection film and the card substrate while using a less strong adhesive between the cover sheet and the protection film. However, delicate formulation of adhesives is practically difficult and the prior art suggests nothing for the measure of selecting such adhesives.
The foregoing problem can substantially be overcome by introducing a technic of a so-called "pseudo-adhesion" formed at the boundary between two thermoplastic resin layers as proposed by the present inventor, for example, in Japanese Utility Model Laying Open No. Sho 61-204729 entitled as "Film with support lead". As described specifically latter, when a thermoplastic layer is extruded under melting over another kind of thermoplastic layer, two layers are adhered by "wetting" and bonded with each other after cooling. The two thermoplastic layers adhere to each other with an adequate bonding strength but can be released relatively easily. We call such a peculiar adhesion as "pseudo-adhesion" and this adhesion can be controlled optionally by varying the melting temperature of the resin and by selecting the combination of two resin materials for the layers.